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Journal of Forestry Research, 12(4): 250-252 (2001)
Concepts of keystone species and species importance in ecology
LU Zhao-hua
(Institute of Restoration Ecology, Chinese University of Mining and Technology, Beijing 100083, P. R. China)
MA Ling, GOU Qing-xi
(Academy of Forest Resources and Environment, Northeast Forestry University, Harbin 150040, P. R. China)
Abstract: This paper discussed the keystone species concept and introduced the typical characteristics of keystone species
and their identification in communities or ecosystems. Based on the research of the keystone species, the concept of species
importance (SI) was first advanced in this paper. The species importance can be simply understood as the important value of
species in the ecosystem, which consists of three indexes: species structural important value (SIV), functional important value
(FIV) and dynamical important value (DIV). With the indexes, the evaluation was also made on species importance of arbor
trees in the Three-Hardwood forests (Fraxinus mandshurica, Juglans mandshurica, and Phellodendron amurense) ecosystem.
Keywords:Species importance; Keystone species; Structural important value; Functional important value;
Dynamical important value
CLC number:$718.5
Document code: A
Article ID: 1007-662X(2001)04-0250-03
Introduction
Concept of keystone species
Keystone species concept has been focused by ecologists and conservation biologists since its introduction by
Robert T. Pain in 1969. By now, however, identification of
the keystone species in communities or ecosystems is not
effective, especially in nature, for example, in forests and
grasslands, even in a functional group. Many scientists
focused on keystone species researches. Tanner and
Hughes (1994) applied a method of sensitivity analysis of a
matrix of transition probabilities to quantify the relative importance of each species in a coral reefs community at
Heron island, Australia, and they successfully identified
"keystone" species (species group) in complex assemblages, but then this method is difficult to be applied in
territory ecosystems. Traditional perturbing experiment can
only be applied in a simple ecosystem or semi-artificially
controlled ecosystem. So far, we have not found a real
objective, comprehensive and quantitative method to identify keystone species in a natural ecosystem. The problem
is how to define and quantify species "importance" in an
ecosystem or community. Species importance concept can
help us understand and quantitatively analyze species
roles, and then identify and evaluate the keystone species
in nature.
The term of keystone species was first introduced by
Robert T. Pain in 1969, and originally applied to a top
predator. The keystone species was defined as: The species composition and physical appearance in a community
or ecosystem were greatly modified by the activities of a
single native species in the food chains. These individual
populations are the keystones of the community. The
structure, integrity of the community and its unaltered persistence through time are determined by their activities and
abundances (Pain 1969). Walker (1992) called those species that are important to an ecosystem "driver", and other
species -- "passengers". Jones et aL (1994) and Lawton
(1994) gave the concept of ecosystem engineers, considered that some species directly or indirectly affect other
species, by causing physical state changes in biotic or
abiotic materials, and thus lead to changes of ecosystem
structure and function. Keystone species play a critical role
in determining community structure. Removal of keystone
species causes massive changes in species composition
and other ecosystem attributes. For instance, removal of
sea otters leads to an increase in sea urchins (Strongylocentrotus sp.), and hence leads to the disappearance of
kelp beds, that in turn changes wave action and siltation
rates, with profound consequences for other inshore flora
and fauna (Estes and Palmisano 1974). In this familiar
example, the species sea otter that was traditionally regarded as the keystone has major effects because it
changes the impact of one engineer (urchin) on another
(kelp), with knock-on effects on other species in the web of
interactions.
Later, the term of keystone species has been broadly
Foundation item: The paper was supported by science foundation of
Changbai Mountain Open Research Station, Chinese Academy of Sciences and Heilongjiang Natural Science Foundation (C00-0I).
Biography: LU Zhao-hua (1963-), male, Ph. Doctor, professor in Institute of Restoration Ecology, Chinese University of Mining and Technology, Beijing 100083, P. R. China.
Received date: 2001-08-1 l
Responsible editor: Zhu Hong
251
LU Zhao-Hua et al.
applied to many species at many tropic levels, and we can
divide the usages of keystone species into five types (Ta-
ble 1).
Table 1. Cate~loriesof presumed keystone and the effects of their effective removal from a system
Keystone
Effect of removal
Organisms
category
Predator
Increase in one or several predators/consumers/ competitions, which subse- Starfish (Paine 1966, 1969), sea otters (Dug-
. . . . . . . . . . . . . -q-u-ent/_y_e_xt!rpa_tes
_
_s_e_v_
er.a.I_p_r_e_y_/oc_mp_et_it_o_r-s_p_ec_i_e_s. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prey
_gin__s_1_980_)_,_f!r_e_ants _(Ris_c_hand _C_a_
r_ro!l_1982)__.
Other species more sensitive to predation may become extinct; predator popula- Arctic hares (Holt 1977), snowshoe hares
. . . . . . . . . . . . . t io_n__ma_y_cr_a_s_h_
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .(Holt
. . . . . .1984)
..........................
Plant
Extirpationof dependent animals, potentially including pollinators and seed dis- Hummingbird pollinators, mammalian dis-
. . . . . . . . . . . . . -p-ers_e_r_s
................................................................
Link
. . . . . . . . . . . . . _quent ~_o_ss
e_s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modifier
]3ers_e_rs__(W_!lco_x_a_n_d_
_M_
urph_y_l_9_8_
5)_. . . . . . . . . . .
Failure of reproduction and recruitment in certain plants, which potential subse- Palmnuts,figs, nectar (Gilbert 1980; Terborgh
)786_k. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Loss of structures/materials that affect habitat type and energy flow; disappear- NorthAmerican beaver (Naiman et al. 1986);
ance of species dependenton particular succession habitats and resources
As used by Pain and many other ecologists, there are
two typical characteristics of keystone species. First, their
presence is crucial in maintaining organization and diversity of their communities and ecosystems. Second, it is
implicit that these species are exceptional, relative to the
rest of community in their importance.
The best way to identify keystone species in a community or ecosystem may be perturbation experiments,
whereby, the candidate keystone species are removed and
the responses of a predefined assemblage of species are
monitored. These tests require adequate experimental
replication and careful definition of the relevant assemblage (MacMabon et aL 1978), and time scales should be
considered.
About the importance of species in community, Mill
(1993) advocated the concept of "community important
value" and defined the community importance of a given
species as the percentage of other species lost from the
community after its removal, thus, we can draw out the
relative community importance value of each species in the
community.
How to understand the effects of species in an ecosystem or community, John H. Lawton summarized four hypotheses: First, the redundant species hypothesis, it suggests that there is a minimal diversity necessary for proper
ecosystem functioning, but beyond that, most species are
redundant in their roles (Walker 1992; Lawton et aL 1993).
Second, the rivet hypothesis, which suggests that all species make a contribution to the performance of an ecosystem (Ehrlich et aL 1981), this hypothesis takes species as
rivets holding a complex machine together. Third, the idiosyncratic response hypothesis, it suggests that ecosystem
function varies when diversity changes, but the magnitude
and direction of change is unpredictable. Finally, the null
hypothesis, that ecosystem function is insensitive to species deletions or additions.
Brazilian termite (Redford 1984)
Concept of species importance in ecology
Species is the basic structural unit of ecosystem. The
structure, function and dynamics of ecosystem are closely
related to its composed species, especially plant species,
and it is the plant species that construct the ecosystem
structure, fix sun energies, ensure matters circulation,
present dynamics and provide ecosystem inner environment. But the role and acting intensity of each species in
the ecosystem are different. How to clearly and objectively
understand the species attributions and quantitatively
measure their contributions in the ecosystem is fundamental and keystone for us to study the ecosystem.
Species importance is a common performance of species in the ecosystem. It is constructed by three indexes:
structural important value (SIV), functional important value
(FIV) and dynamical important value (DIV). Species structural important value is composed of important values of
main-layer species, succession-layer species and generation-layer species. The species functional important value
is composed of five measuring values: species biomass,
species biomass energies, species net primary productivities, species growth production, and species leaf litter
production. And species dynamical important value is calculated by Horn's Markov model (1975a, 1975b). The results of species importance of arbor trees in the
Three-Hardwood forests ecosystem is as followed (Table
2).
Discussion
In nature, ecosystem is changing constantly, and each
succession stage has its own keystone species, so keystone species are varying as their ecosystem changes.
The role of species in ecosystem processes is mainly reflected in three aspects: structural importance, functional
Journal of Forestry Research, 12(4): 250-252 (2001)
252
importance, and dynamic importance. However, the impacts of species structural importance, species functional
importance and species dynamic importance on the ecosystem are not equal.
Table 2. Species importance ISl/of arbor trees in the Three-Hardwood forests ecosystem
Tree species
Fraxinus mandshurica
Juglans mandshurica
Phellodendron amurense
Ulmus japonica
Acer mono
Tilia amurensis
Betula platyphylla
Populus davidiana
SIV
SIV Order
FIV
FIV Order
DIV
DIV Order
SI
SI Order
20.18
25.61
13.57
21.64
12.99
2.28
1.58
2.10
3
1
4
2
5
6
8
7
12.76
62.83
11.53
3.30
1.61
2.43
1.65
3.90
2
1
3
5
8
6
7
4
26.86
18.43
6.96
29.05
17.67
0.44
0.30
0.28
2
3
5
1
4
6
7
8
19.93
35.62
10.69
18.00
10.76
1.72
1.17
2.10
2
1
5
3
4
7
8
6
Notes: SIV---structural important value; FIV---functional important value; DIV---dynamical important value; SI---species importance.
The best way to identify keystone species would be perturbation experiments whereby the candidate keystone are
removed and responses are monitored. But such tests
require adequate experimental replication, careful attention,
and time scales should be considered. Traditional perturbing experiments may be only applied to a simple system or
semi-artificially controlled system; it is difficult to be applied
in natural ecosystems such as forests and grasslands.
Lack of a unit, objective and quantitative criterion of
identifying keystone species leads to different keystone
species in the same ecosystem by different scientists at
the same time.
Redundancy is not surplus, thus this surplus is a kind of
ecological redundancy. And just the redundancy results in
stability, persistence and integrity of ecosystem.
Keystone species concept overstates the impacts of
keystone species on many other species. In fact, several
non-keystone species may have a greater contribution
than a single keystone species.
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